1. Field of the Invention
The present invention relates to a backlight unit most suitable for a backlight of a liquid crystal display device and a liquid crystal display device including the same.
2. Description of the Related Art
FIG. 21 is a schematic sectional view illustrating an example of a conventional passive (simple matrix type) liquid crystal display device. A liquid crystal display device 300 according to the example includes a transmissive or transflective liquid crystal display unit 320 and a backlight 310 arranged on the rear side of the liquid crystal display unit 320 (for example, refer to the Patent Documents 1 and 2.).
In the backlight 310, light emitted from a long light source 313 such as a cold cathode fluorescent lamp (CCFL) is incident on an incident surface (a side surface) 312a of a plate-shaped light guide plate 312 and emits from an emission surface (an upper surface) 312b of the light guide plate 312 that is opposite to the liquid crystal unit 320.
A reflection member 317 composed of a plurality of protrusions or dot-shaped plane patterns, which are white or reflective, is formed on a surface (a lower surface) 312c opposite to the emission surface 312b of the light guide plate 312, thereby providing light reflection property.
A diffuser sheet 314 is arranged on the emission surface 312b of the light guide plate 312, and two prism sheets 315 and 316 are sequentially disposed on the diffuser sheet 314, thereby providing the diffusivity of light and the uniformity of brightness distribution. Each of the prism sheets is obtained by forming a series of protrusions 318 being triangular shapes in section and a series of grooves 319 being wedge shapes in section on a layer formed on a substrate as illustrated in FIG. 22. In the two prism sheets 315 and 316, the direction, where the edge lines of the protrusions 318 of one prism sheet are extended, is at an angle of 90° with the direction, where the edge lines of the protrusions 318 of the other prism sheet are extended (that is, the structures of two prisms are orthogonal to each other). Therefore, a light component in a certain direction among the light components emitted from the emission surface 312b of the light guide plate 312 passes through the prism sheet 315, is collected at a viewing angle in a certain angle range (for example, 0° to 70°), and is emitted as emitted light L21. Furthermore, a light component in a different direction passes through the prism sheet 316 of the other side, is collected at a viewing angle in a certain angle range (for example, 0° to 70°), and is emitted as emitted light L22.
Recently, the use of a white light emitting diode (LED), which is a point light source, as a light source for a backlight is suggested.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 11-500071
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 10-169922
However, the conventional backlight 310 has a poor utilization efficiency of light. For example, though light is emitted from the light source 313 to the light guide plate 312, some light components leak from the lower surface 312c of the light guide plate to the outside. Since the conventional backlight 310 has a large number of parts, the thickness thereof tends to increase, and in addition the price of the backlight 310 becomes expensive. Furthermore, according to the conventional art, a thin and low cost backlight obtained by improving the utilization efficiency of the light emitted from the light source, which is capable of uniformly and brightly illuminating the display region (an illuminated region) of the liquid crystal display unit 320, is not realized.
Moreover, since the conventional liquid crystal display device 300 includes the above-mentioned backlight 310, the liquid crystal display device 300 has a larger thickness, costs much, and has large power consumption.